Various types of optical spectrometers are in use for such purposes as atomic emission spectroscopy, atomic absorption spectroscopy and astronomy. A complete system generally consists of a source of radiation, a spectrometer for separating and detecting individual spectral components, and a data station for processing the information from the spectrometer. The radiation source, for example, may be a system for injecting a test sample into an inductively coupled plasma where the atomic species in the sample are excited to radiate characteristic atomic emission. As another example, a sample is evaporated in a graphite furnace where the gaseous sample absorbs certain frequencies of the incident radiation to provide atomic absorption lines. Similarly, astronomical sources provide atomic emission and absorption lines.
A type of spectrometer of particular interest herein involves sequential measurement utilizing a monochromator in which a grating or prism is rotated to direct a narrow portion of the spectrum to a detector. The angle is adjusted to correspond to the different emission (or absorption) lines of the elements. A single detector is used, either a solid state detector or a photomultiplier tube. The measurement process involves rotation of the grating with measurements at a fixed location corresponding to grating angles appropriate to the atomic emission lines.
Sophisticated monochromators, particularly of the type used for quantitative analysis of atomic elements in samples injected through an induction coupled plasma, are controlled by microprocessors and personal computers. Such a system is typified by a Model Plasma II emission spectrometer sold by The Perkin-Elmer Corporation, Norwalk, , and described in U.S. Pat. No. 4,779,216 by Collins, assigned to the assignee of the present application. A stepper motor orients a grating with respect to the slit of the detector to locate any selected portion of the spectrum for measurement of the intensity of that portion. A dedicated microprocessor provides suitable signals to the motor for selective orientation in relation to wavelength. The microprocessor also receives the intensity signal from the detector, and provides data in the form of spectral intensity vs. spectral position. In practice, scanning signals are provided to the motor to sequentially scan the spectrum in a series of steps.
In order to allow a reasonably fast scan, signals to the motor are such as to scan in spectral windows which are just wide enough to encompass each of the selected spectral bands with some margin. The motor scans through all steps in a window, and then moves quickly to the next window before scanning in steps again, and on to the next window, etc., for the whole series of spectral bands.
Calibration of the instrument typically is accomplished by running a standard element of known concentration, and a background or blank run with no sample before running the unknown samples. This provides two points for the calibration.
For further accuracy, it is necessary to compensate for fluctuations in the source, such as variations in the amount of sample material injected into a source plasma. For this purpose, standardization is generally effected by measuring intensity of a spectral line of a reference element added in a known quantity in the source. During each run, or selectively between the runs for unknown elements in a sample, an intensity of a reference spectral line is determined. A ratio of a sample line intensity to that of the reference line provides a standardized measure of intensity and thereby quantity of the unknown atomic species. This method, which has been in extensive use for many years, is sequential standardization. A detrimental problem is that results are susceptible to interference from extraneous radiation as from electrolytes added to sample liquids injected into the plasma. In such case, recovery, i.e. measured intensity, may be unpredictably low.
An alternative standardization method was refined by Myers and Tracy as reported in "Improved Performance using Internal Standardization in Inductively-coupled Plasma Emission Spectroscopy" by S. A. Myers and D. H. Tracy, Spectrochimica Acta, 38B, 1227-1253 (1983). Known as simultaneous standardization, the method involves splitting out a fraction of the input beam from the source. This split portion is further split, and one half is used for measurement of a standard line intensity. The other half is used for measurement of background radiation which is subtracted from the standard and sample line intensities. A "Myers-Tracy" ratio is calculated as the ratio of the corrected intensities of the sample and the standard. This standardization method has significantly improved average accuracy (recovery), since the compensation measurements are made simultaneously, and is used in the Perkin-Elmer Plasma II. However, there often is a considerable scatter in individual results, i.e. poor accuracy.
Therefore an object of the present invention is to provide a novel method and an apparatus for standardizing spectral line intensities to improve compensation for source fluctuations. A further object is to provide a novel method with an optical spectrometer for effecting spectral measurements having improved recovery and accuracy. Another object is to provide an improved optical spectrometer for achieving high accuracy and precision in quantitative measurements for atomic elements.